Spezza sustained in the injury after a hit by forward Adam Lowry during the third period of a 4-3 loss to the Winnipeg Jets on Thursday.
"Right now we really don't have a time frame on it, but it's more than days," Ruff said. "I'll just be honest. It's an upper body injury that doesn't have anything to do with anything old.
"It's not a concussion. It's going to take a week or two, maybe longer."
Spezza scored eight goals and set up 23 others in 45 games this season. The 33-year-old Ontario native has 309 goals and 534 assists in 888 career contests with the Ottawa Senators and Stars.
One of the options to replace Spezza might have been veteran forward Jiri Hudler, but he missed a good portion of the season due to illness and has recently been out of the lineup again.
"Well, he (Hudler) just missed the last two days with a family leave, hasn't skated, so now he's got himself in a tough situation again," Ruff said. "It's a decision we're going to have to make. Give him a skate today and go from there, but I think the illness put him way behind the eight-ball."